michie (sounds like Mickey) — an attribute macro that adds memoization to a function.

Table of contents

1. Features
2. Non-features
3. key_expr
4. store_type
5. store_init
6. Type requirements
   1. General bounds
   2. Store bounds
7. Generic functions
8. Functions that take no input
9. How it works
10. Why must key_expr be provided
11. Support and feedback

Features

• Supports
  • Plain functions
  • Generic functions
  • Functions in impl blocks
  • Functions in trait implementation blocks
  • Functions that are default trait implementations
• Thread safe
• Expansion depends on only std
• Hygienic
• Supports recursive functions
• Bring your own store

Non-features

• Caching features: this crate does not provide a caching mechanism other than some trivial implementations. It allows you to bring your own.
• "Blazingly fast": this crate aims to provide a simple and easy-to-use means of memoizing a function. If you actually really require micro-optimized memoization then you'd most likely have to implement it yourself.

key_expr

In each invocation a key is obtained. It is used to query the function's cache store for a possible hit. An expression that evaluates into a key must be provided via the key_expr argument. The expression may use bindings from the function's parameters. In the following example the key_expr is simply the name of the only parameter.

use michie::memoized;
use std::collections::HashMap;
#[memoized(key_expr = input, store_type = HashMap<usize, usize>)]
fn f(input: usize) -> usize {
    // expensive calculation
    # unimplemented!()
}

store_type

A concrete store type must be either provided via the store_type argument or inferred from the store_init (next section).

The provided type must implement [MemoizationStore]. Implementations of [MemoizationStore] for [BTreeMap] and [HashMap] are provided. In the following example, [BTreeMap] is provided as the store:

use michie::memoized;
use std::collections::BTreeMap;
#[memoized(key_expr = input, store_type = BTreeMap<usize, usize>)]
fn f(input: usize) -> usize {
    // expensive calculation
    # unimplemented!()
}

store_init

By default, the store is initialized via [Default::default()]. Different initialization may be provided via an expression to store_init:

use michie::memoized;
use std::collections::HashMap;
#[memoized(key_expr = input, store_init = HashMap::<usize, usize>::with_capacity(500))]
fn f(input: usize) -> usize {
    // expensive calculation
    # unimplemented!()
}

Type requirements

Bounds apply to the key type and the function's return type. Some are from the general instrumentation and others are via the store type's implementation of [MemoizationStore].

General bounds

The following apply to the key type and to the function's return type:

• [Sized]: for one, the instrumentation stores the key in a let binding.
• 'static: key and return values are owned by a store which is owned by a static.
• [Send] and [Sync]: for parallel access.

Store bounds

Another source of bounds on the key type and the return type is the implementation of [MemoizationStore] for the store type.

Generic functions

Be mindful of the type requirements when using on a generic function:

use michie::memoized;
use std::hash::Hash;
use std::collections::HashMap;
#[memoized(key_expr = input.clone(), store_type = HashMap<A, B>)]
fn f<A, B>(input: A) -> B
where
    A: 'static + Send + Sync // bounds from instrumentation
        + Eq + Hash // store-specific bounds
        + Clone, // used in this function's `key_expr`
    B: 'static + Send + Sync + Clone // bounds from instrumentation
        + From<A>, // used in this function's body
{
    input.into()
}

Functions that take no input

Functions that take no input are good candidates for compile-time evaluation, which is usually preferred over runtime caching (such as this crate provides). Nonetheless, some functions cannot be evaluated at compile time. A reasonable key_expr for a function that takes no input is ():

use michie::memoized;
use std::collections::HashMap;
#[memoized(key_expr = (), store_type = HashMap<(), f64>)]
fn f() -> f64 {
    // expensive calculation
    # unimplemented!()
}

How it works

The original function expands into something similar to this:

fn f(input: Input) -> Output {
    static STORE = Mutex::new(#store_init);
    let key = #key_expr;
    let store_mutex_guard = STORE.lock().unwrap();
    let attempt = store_mutex_guard.get(&key);
    drop(store_mutex_guard);
    if let Some(hit) = attempt {
        return hit;
    } else {
        let miss = #original_fn_body;
        let miss = STORE.lock().unwrap().insert(key, miss);
        return miss;
    };
}

Why must key_expr be provided

The only conceivable default key_expr is the entire input. For example, for a function signature:

fn f(a: usize, _b: usize) -> usize

the default key_expr would be (a, _b). Two potential problems: some parameters might not satisfy bounds on the key type. Also, the resulting key might be a supervalue of the input of the actual calculation. To explain the latter problem, here is an example:

use michie::memoized;
use std::collections::HashMap;
// pretend that `key_expr` is omitted and that this is the default
#[memoized(key_expr = (a, _b), store_type = HashMap<(usize, usize), usize>)]
fn f(a: usize, _b: usize) -> usize {
    // the actual calculation uses a subvalue of the input — only `a`
    # a
}
f(0, 0); // expected miss because it's the first invocation
f(0, 1); // avoidable miss!

If an accurate key_expr = a had been provided, the second execution would have been a hit. To summarize, key_expr is mandatory in order to encourage proper consideration of it.

Support and feedback

In the GitHub Discussions.